This invention relates to a polishing tape to be used for the precision finishing of a surface of a magnetic hard disk substrate. In particular, this invention relates to such a tape with a very fine surface roughness (Ra) and a method of texturing a substrate surface by using such a polishing tape.
With the recent development in the so-called high-tech industries centered around the electronics industries, memory capacity of magnetic disks is becoming higher and there is an increasing demand for high precision in the finishing of disk substrate surfaces. If a magnetic head is stopped on a magnetic disk thus structured, however, the magnetic head may be adsorbed to the magnetic disk due to the water component or a lubricant adsorbed to the disk surface. In order to prevent such occurrence of adsorption, it has been known to carry out a texturing process to form fine concentric protrusions and indentations on the surface of magnetic hard disk substrates in the circumferential direction of the substrate. The texturing process is usually carried out by using a polishing tape obtained by coating the surface of a backing material (say, of polyester) with abrading particles (say, of white molten alumina) or a slurry obtained by dispersing such abrading particles in a liquid.
Prior art abrading particles such as prior art alumina particles for abrading are not uniform in sizes or shapes, there being great variations and some of the larger particles protruding from the polishing surface of the tape. If a target surface is polished by means of such a tape, large particles tend to grind the target surface too deeply, leaving undesirably tall scratch marks on the surface.
As the recording density on the magnetic disk is increased, the height of the magnetic head over the magnetic disk must be reduced in order to improve the signal sensitivity at the time of recording and reproduction, reducing the distance of separation therebetween. If there are protrusions sufficiently high on the substrate surface, however, the magnetic head may collide with such a protrusion (an event referred to as the xe2x80x9chead hitxe2x80x9d). If the texturing is carried out intentionally insufficiently in order to prevent the generation of protrusions, however, the magnetic disk will end up being too smooth on the surface and the adsorption to the head will result, as described above.
Even if the texturing is carried out by using a liquid slurry serving as free abrading particles, similar problems are encountered as long as there are variations in the sizes and shapes of the abrading particles. If the texturing process is done only lightly in fear of the occurrence of head hit, one again faces the problem of adsorption of the disk to the magnetic head.
Japanese Patent Application 8-88954 disclosed a type of tape produced by applying a adhesive on the surface of a plastic tape and planting piles of 6-nylon, 66-nylon, vinylon or polyester thereon. Since plastic tapes have a uniform thickness and a flat surface, piles can be planted uniformly and at a high density. Thus, a very fine and uniform texturing process is possible with such a polishing tape. There is a problem, however, with this type of polishing tapes in that the planting of the piles becomes difficult if the piles are too short. For this reason, there have been attempts to produce a woven polishing tape by combining longitudinally and transversely extending fibers.
In the meantime, there has been a demand to increase the memory capacity of the disks. A fine surface roughness can be attained by using tapes with planted piles or woven tapes if fibers with small diameters are used. Recently, however, the surface roughness is coming to be required to be even smaller than possible by reducing the thickness of the fibers. In other words, the era of tapes with planted piles and woven tapes seems to be coming to an end. In addition, tapes of these kinds require the use of a liquid slurry, there remaining the problem of producing unwanted protrusions on the target surface or having the abrading particles themselves embedded in the target surface. Another problem with the use of a liquid slurry is that the debris particles resulting from the grinding are carried around throughout the polishing process. Thus, the target surface may be damaged by such debris. Scratches produced thereby and the embedded debris themselves are both likely to cause the head hit. Still another problem of such prior art texturing processes is that the debris produced by the grinding must be removed afterwards and hence that it is time-consuming.
It is therefore an object of this invention to provide a polishing tape with which smaller surface roughness can be attained in response to the recent demand for higher capacities of hard disks while obviating the problem of head hit.
A polishing tape embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising a backing material and a foamed material such as polyurethane foam. The foamed material is elastic and some of its gas holes formed inside but near one of its surfaces are exposed externally. Because of its elastic nature, it can be deformed even if there are abnormally large abrading particles contained in a liquid slurry and such abrading particles do not become embedded in the target surface being textured. These externally exposed gas holes can also serve to absorb the debris generated by the texturing and to protect the target surface from being scratched thereby. Such a tape is used for a texturing process while a liquid slurry containing abrading particles is dropped, the disk substrate is rotated and the tape is pressed against the target surface.